Electron discharge device



Jan. M N41; K. STEIMEL ETAL ELECTRON DISCHARGE DEVICE Filed Oct. 20,1939 [I III III IN VEN TORS LR K E% M MC R m m SM T L fl C AA KO J Y BPatented Jan. 14, 1941 UNITED STATES PATENT OFFICE Berlin, Germany,

assignors to Telefunken Gesellschaft fiir Drahtlose Telegraphic m. b. H,Berlin, Germany, a corporation of Germany Application October 20, 1939,Serial No. 300,366 In Germany September 27, 1938 3 Claims.

The invention relates to an electric discharge tube of the pentode typehaving at least a thermionic cathode, three grid electrodes surroundingthe cathode, and an anode. Tubes of this type 5 are so esigned that theinnermost grid neXt to the cathode serves as control electrode, the following grid is the screen grid, and the third grid acts as a suppressoror retarding grid for suppressing the exchange of secondary electronsbe- IO tween the anode and the screen grid. The screen grid provides adecoupling or screening between the anode and the control grid andassures a decrease of the capacity between the said two electrodes.Usually the grids are wire helices car- 15 ried on one or more supportor grid side rods, and

the side rods are in alignment.

If such tubes are employed in circuits with variableamplification,thefield of the anode and screen grid through the controlgrid is often made nong9 uniform for the purpose of obtaining a flatcurve of the plate current characteristic in reference to the controlgrid potential. The non-uniform field is brought about in general byvarying the pitch of the grid helix, and it is customary to have the 25largest pitch of the grid at the middle of the cathode and gradually todiminish the pitch towards the ends of the cathode. If the pitch of thecontrol grid is non-uniform, often the screen grid is also non-uniform,and in certain cases the 30 retarding grid has a non-uniform pitch. Ifmore than one grid is of non-uniform pitch, the arrangement is generallysuch that both the screen grid and the retarding grid have, like thecontrol grid, the largest pitch at the middle of the 35 cathode. In manycases the screen grid cannot be as closely wound with uniform pitch ascould be desirable for the shielding or capacitive decoupling betweenthe control grid and the cathode. If the screen grid has a fine mesh, itabsorbs much current, which causes a strong rustling noise in the tube.In order to reduce this noise as much as possible, the currentdistribution between the screen grid and the anode should be such thatlittle current goes to the screen grid. This current dis- 4 tributionmay be obtained by winding the screen grid with wide spacing; forinstance, by choosing the pitch of the screen grid winding twice aslarge as that of the control grid, so that the thickness of the screengrid wire is approximately only one- 50 tenth of the pitch. In oneexample of construction the pitch of the control grid was 0.27 mm., thepitch of the screen grid 0.6 mm. and the thickness of the screen gridwire was 0.05 mm.

If the pitch of the control grid winding at the m middle of the cathodeis wider than at the ends,

the pitch of the screen grid should be in the same proportion,otherwise, the middle of the screen grid would absorb an inadmissiblylarge amount of current. In one example of construction the pitch of thecontrol grid which was 0.27 mm. 5 at the ends of the grid was increasedin the center thereof to 6.5 mm; the screen grid was wound at the endswith a pitch of 0.6 mm. and in the center with a pitch of 1.0 mm. Thechange of the pitch was in approximately the same propor- 10 tion inboth grids.

The retarding or suppressor grid cannot be expected to compensate forthe poor shielding action or the large pitch part of the screen grid.The retarding grid should be constructed in the same manner at the placewhere the screen grid has the large pitch, since otherwise anexcessively dense space charge would collect in front of the retardinggrid; therefore if the retarding grid is of nonuniform pitch, the centermust be of such a large pitch that the suppressor grid contributeslittle to the shielding, or ii of uniform pitch, it must be of thislarge pitch throughout its length.

The principal object of the invention is to provide a tube particularlyof the suppressor grid type, which has desirable characteristics due tononuniform pitch and also good shielding between the anode and controlgrid circuits.

In accordance with the invention the increase incapacity between thecontrol grid and the anode 3 resulting from the use of wound grids ofnonuniform pitch or of uniform but large pitch is compensated for orcounteracted by interposing between the retarding grid and the anode atleast one shielding electrode which extends across the grid rod sectorof the electrode interspace in the electron shadow of the grid rods,where the inter electrode space is not filled with the dischargecurrent.

The invention will best be understood by referonce to the followingdescription taken in connection with the accompanying drawing whichshows schematically examples of construction in accordance with thepresent invention, ancl'in which Figure 1 is a cross-section through oneform of pentode with shields separate from the suppressor grid; Figure 2is a cross-section of a tube with the shields attached to the suppressorgrid and in effect constituting grid rods; and Figure 3 a longitudinalsection of the tube shown in Figure 1.

Referring to the drawing, Figure 1 represents schematically across-section and Figure 3 a longitudinal section through an electrodesystem having a cathode l, a control grid 2, a screen grid 3, and

a suppressor or retarding grid 4. These grids are preferably fine wirehelices, oval in cross-section, and each helix is fixed to two supportsor side rods at the ends of the largest diameter of the helix. The gridsare coaxial with and surround the cathode and are surrounded by an anode5, which may be oval or circular in cross-section. The screen grid 3 iswound at least in part with a comparatively large pitch; The dischargecurrent will be distributed in a non-uniform fashion over thecircumference of the cathode and is concentrated to a certain extent bythe oval shape of the grids and by the beam forming action of the gridside rods so that two electron beams are produced which extend inopposite directions from the cathode and perpendicular to the plane ofthe side rods of the grids.

In accordance with the invention two shield electrodes 6 are interposedbetween the anode 5 and the retarding grid 4. These shields, which arecurved to match the curvature of the electrode system, are opposite thegaps or more open portions of the grids and substantially cover thegaps, as shown in Fig. 3, and are of such width that the shields extendacross only the spacial sector which is defined by the electron shadowsof the grid rods and which are between the sec tors occupied by the: twoelectron beams. The shields are given a constant potential, and may forinstance, be connected to the cathode or to the retarding grid insidethe tube. Since the shield electrodes are situated outside the dischargepath, they have practically no influence upon the discharge. On theother hand, they cut off entirely the lines of force of theelectrostatic field between the anode 5 and the control grid 2 in thespecial sectors covered by said shields. As a result, the capacitybetween the anode and the control grid will be reduced, and thedetrimental increase in capacity coupling of the anode and control gridcircuits due to the non-uniform winding of the screen grid iscompensated for and practically eliminated.

Figure 2 shows a cross-section through an electrode system containingthe same parts as the system shown in Figure 1. The shielding sheetmetal electrodes 6 are connected in this case directly to the supportrods of the retarding grid, and therefore, have the potential of theretarding grid. It is obvious that the support rods may be omitted, andthe shielding electrodes alone may act as supports for the winding ofthe retarding grid.

A shield electrode in the electron-free space between the retarding gridand the anode can obviously be provided even where the electrode formsdiffer from those shown as examples of construction. If, for instance,the grids are each supported at one side only by a supporting rod, it isonly at this side that the shielding electrode is needed. On the otherhand, three or four electrodes may be distributed over the circumferencewhere through an identical number of grid rods at each grid an equalnumber of discharge-free spaces are provided.

The arrangement of the shielding wall between the retarding grid and theanode is to be preferred over the arrangement in which the shields arearranged at the cathode side of the retarding grid, since at theaforesaid place the discharge will practically not be influenced at allwhile the shield inside the retarding grid could produce electronicspace charges in certain cases.

The capacity between the anode and the control grid may be reduced ifthe anode is made of sheet metal segments which extend only across thesector in which the discharge flows, but this arrangement has somedisadvantages, since on the one hand the shielding of the dischargespace towards the outside is no longer as favorable and disturbances mayreadily occur due to exterior fields, and, on the other hand, themechanical stability of such an anode composed of segmentlike metalsheets will be lower. It can also be said that in this case thestructure of the system will be more complicated, since the type ofanode which acts as a support for the entire system is not present.

We claim:

1. An electron discharge tube comprising a thermionic cathode, an anode,three successive grids between said cathode and anode, each of saidgrids having a side rod and said grids having their side rods inalignment between said cathode and said anode, the first grid next thecathode having a portion more open than the remainder of the grid, thesecond grid from the cathode having a portion more open than theremainder of said grid, said more open portion of said sec- 0nd gridbeing opposite the more open portion of said first grid, and a sheetmetal shield electrode in the interspace between said second grid andsaid anode opposite the more open portion of said second grid andextending transversely of the plane of said grid rods and across onlythe spacial sector defined by the electron shadow of the grid rods.

2. An electron discharge tube comprising a thermionic cathode, an anode,three helical grids of different diameters and oval cross-section, eachhaving two side rods at opposite ends of the major axis of said grid,said grids being mounted between said cathode and anode with their siderods in a common plane, the first and second grids from said cathodehaving a variable pitch in the same proportion and with the most openportions of said grids-opposite each other, and a sheet metal shieldelectrode between said second grid and said anode in the electron shadowof the side rods of said grids and extending transversely of the planeof said grid rods opposite the more open portion of said second grid andacross only the spacial sectors defined by the electron shadows of saidgrid rods.

3. An electron discharge device comprising a thermionic cathode, ananode, a helically wound control grid next to the cathode having aportion of large pitch, a screen grid next said control grid and havingopposite the large pitch portion of said control grid a portion ofequally large pitch, a suppressor grid next to the anode and havingopposite the large pitch portion of said screen grid a portion ofequally large pitch, each of said grids having a side rod and said gridsbeing mounted with their side rods in alignment, and a sheet metalshield electrode between said anode and said screen grid opposite thelarge pitch portion of said screen grid and outside the grid rods of thesuppressor grid, said shield electrode being in the electron shadow ofsaid grid rods and extending transversely of the plane of said rodsacross only the spacial sector defined by the electron shadows of saidgrid rods.

KARL STEIMEL. J OACHIM SCHEEL.

